Analog or microwave-photonic optical links have found widespread application in numerous sectors of communications. All modem cable-television networks use high-performance linear lasers and optical fiber to deliver the entire spectrum of analog video and subcarrier-modulated digital data to and from small groups of subscribers. See, e.g., T. E. Darcie, M. R. Phillips, “Lightwave Video Transmission,” Chapter in Optical Fiber Telecommunications IIIA, Academic Press, NY (1997). Other broadband access or last-mile networks, like the fiber-to-the-premises networks being deployed by telephone carriers throughout the world, also use analog optical links for transmission of broadband RF-modulated signals. See, e.g., T. H. Wood, G. C. Wilson, R. D. Feldman, J. A. Stiles, “(FTTH) system providing broad-band data over cable modems along with analog and digital video,” IEEE Photonics Technol. Lett., Volume 11, Issue 4, pp. 475-477 (April 1999). With the allocation of new high-frequency (e.g. 60 GHz) microwave bands for communications networks, optical links become an essential component in overcoming loss associated with coaxial or waveguide alternatives. Cellular base stations are often connected to remote antennas or groups of antennas by microwave-photonic links. See, e.g., I. Rivas, L. B. Lopes, “Transmitter macrodiversity in radio fibre microcellular networks,” Personal, Indoor and Mobile Radio Comm., PIMRC, Vol. 3, 1-4, pp. 1074-1078 (September 1997). Phased-array microwave and millimeter-wave antennas use microwave-photonic links to deliver phase reference and signal information to and from centralized processors. See, e.g., J. B. Georges, K. Y. Lau, “Broadband microwave fiber-optic links with RF phase control for phased-array antennas,” IEEE Photonics Technol. Lett., Vol. 5, Issue 11, pp. 1344-46 (November 1993). In short, the transmission of microwave information as modulated optical signals through optical fiber has become an essential tool in modern communications networks. As a result, considerable work has gone into maximizing the performance of these links. See, e.g., C. H. Cox III, G. E. Betts, and L. M. Johnson, “An analytic and experimental comparison of direct and external modulation in analog fiber-optic links,” EEE Trans. Microwave Theory and Techniques, vol. 38, pp. 501-509 (May 1990); L. T. Nichols, K. J. Williams, and R. D. Esman, “Optimizing the ultrawide-band photonic link,” IEEE Trans. Microwave Theory and Techniques, vol. 45, pp. 1384-1389 (August 1997).
Impairments introduced by microwave-photonic links (MPLs) include primarily noise and distortion. Noise limits the minimum microwave signal level that can be detected. Linearity limits the maximum microwave signal power that can be transmitted. The difference between minimum and maximum is described by the spur-free dynamic range (SFDR), one of the key figures of merit for an MPL. See, e.g., C. H. Cox, III, “Analog Optical Links: Theory and Practice”, Cambridge (2004). Performance is measured by the ability of the optical link to preserve the fidelity of the transmitted microwave signal, as measured by parameters like the SFDR, composite-triple beat (CTB), carrier-to-noise-ratio (CNR), and link gain. Collectively, these parameters describe the ability of the link to deliver large modulated signals while minimizing noise or interference.
In most systems, the light source (directly-modulated laser or continuous-wave (CW) laser followed by an external modulator) is biased to a linear operating point, and the RF signal is applied symmetrically about this bias point. The DC light intensity, or residual carrier, associated with this bias carries no information, but is responsible for many of the limitations of the link performance. Shot noise and relative-intensity noise (RIN), often the dominant noise sources in high-power links, result directly from the detection of this residual carrier. In systems using optical amplifiers, beating between the carrier and amplified spontaneous emission result in high levels of signal-spontaneous beat noise. Also, the available gain of the optical amplifier is consumed or saturated by this residual carrier, rather than by the desired signal. See, e.g., J. M. P. Delavaux, A Yeniay, B Neyret, C. Hullin, G. R. Wilson, “Multiple-output Er-3+ amplifier for analog and QAM distribution systems,” Optical Fiber Communications Conference, Vol. 3, pp. WDD30-1-3 (2001). The residual carrier is subject to optical nonlinearities (stimulated Brillouin scattering (see, e.g., X. P. Mao, G. E. Bodeep, R. W. Tkach, A. R. Chraplyvy, T. E. Darcie, R. M. Derosier, “Brillouin scattering in externally modulated lightwave AM-VSB transmission systems,” IEEE Photonics Technol. Lett., Vol. 4, Issue 3, pp. 287-289 (March 1992)), nonlinear refractive index (see, e.g., M. R. Phillips, T. E. Darcie, D. Marcuse, G. E. Bodeep, N. J. Frigo, “Nonlinear distortion generated by dispersive transmission of chirped intensity-modulated signals,” IEEE Photonics Technol. Lett., Vol. 3, Issue 5, pp. 481-483 (May 1991)), stimulated Raman scattering) limiting the amount of power that can be transmitted through the fiber. Finally, this residual carrier constitutes most of the power that saturates the photodetector, limiting the maximum signal power. Hence the DC bias required to operate at a linear operating point limits both the low end (noise) and high end (signal power) of the SFDR.
A variety (5 classes) of techniques has been proposed to mitigate the effect of the residual carrier. First, the carrier can be reduced through optical filtering. See, e.g., R. D. Esman, K. J. Williams, “Wideband efficiency improvement of fiber optic systems by carrier subtraction,” IEEE Photonics Technol. Lett., Vol. 7, No. 2, pp. 218-220 (February 1995). This has the same effect as increasing the modulation index (or lowering the DC bias) with commensurate increases in nonlinear distortion.
Second, coherent techniques have been proposed which use heterodyne detection to overcome the linearity associated with low- bias operation of a Mach Zehnder (MZ) external modulator. See, e.g., A. C. Lindsay, “An analysis of coherent carrier suppression for photonic microwave links,” IEEE Trans. Microwave Theory and Tech., Vol. 47, Issue 7, pp. 1194-1200 (July 1999). Unfortunately, the local oscillator generates noise that will offset the low-bias gain, and considerable complexity is added, including a frequency-stabilized laser that is required at the receiver.
Third, the bias can be modulated dynamically in response to the instantaneous magnitude of the RF envelope. See, e.g., U.S. Pat. No. 6,181,453 entitled “Method and apparatus for laser performance enhancement,” and issued on Jan. 30, 2001 to T. E. Darcie and P. P. Ianonne. This can reduce the effective DC level, especially for signals with large peak factors, but the nonlinear mixing between the signal and bias modulation (the square of the signal) creates problematic third-order distortion. Also, while this approach can reduce noise from the residual carrier, the minimum average power remains substantially larger than zero, limiting the ultimate improvement.
Fourth, common-mode RIN (RIN present at the input to a Mach-Zehnder modulator (MZ)) can be cancelled using two fibers and a balanced photodetector. See, e.g., S. Mathai, F. Cappelluti, T. Jung, D. Novak, R. B. Waterhouse, D. Sivco, A. Y. Cho, G. Ghione, M. C. Wu, “Experimental demonstration of a balanced electroabsorption modulated microwave photonic link,” IEEE Trans. Microwave Theory and Tech., vol. 49, pp. 1956-1961 (October 2001). Significant suppression of RIN has been demonstrated, but this does not affect the other challenges associated with the large residual carrier. Also, it has been shown that this technique results in reduced suppression of intensity noise for signals with larger modulation index.
Fifth, low-bias techniques have been explored in which an MZ is operated at lower bias than the conventional quadrature bias point (50% transmission). Unfortunately, this increases the distortion and decreases the signal, limiting the usefulness of the approach. In an attempt to reduce the distortion of this low-bias technique, an approach was explored in which 2 MZs were operated in an anti-symmetric manner with a balanced detector. See, e.g., W. K. Bums, G. K. Gopalakrishnan, R. P Moeller, “Multi-octave operation of low-biased modulators by balanced detection,” IEEE Photonics Technol. Lett., Volume 8, Issue 1, pp. 130-132 (January 1996). Reduction of second-order distortion was demonstrated, but maintaining the appropriate balances was challenging and overall improvement in noise was not demonstrated. The intent was to minimize second-order distortion in a low-biased link to achieve broadband operation. Hence the modulators were biased at an operating point at which the even-order distortion was small.
To summarize, there are 3 main causes of noise: Receiver noise; shot noise; and relative intensity noise (RIN). Shot noise power increases linearly with total received power. Noise from RIN increases as the square of total received power. Receiver noise is independent of received power. While the prior art has been somewhat successful in reducing receiver noise and the effect of RIN, to date and to our knowledge, no practical method for reducing shot noise has been defined and RIN continues to be a problem. It is an object to overcome the deficiencies of the prior art.